(57) A control system for resistance welders of the type in which welding current is supplied to a workpiece through a transformer. The system assures weld quality by monitoring the change in resistance across the weld during the welding operation without the need for auxiliary leads or separate sensing transducers at or near the workpiece and automatically terminates the welding current when a quality weld is made as determined by weld resistivity characteristics. To accomplish this, the system monitors the discrete changes in the characteristics of the current in the primary of the welding transformer which occur as a result of changes in weld resistance. Immunity to line voltage disturbances which can cause changes in current characteristics unrelated to resistance change in the weld is achieved by a modeling circuit which is subjected to the same line voltage disturbances as the actual welder circuit and which simulates the current characteristics of the actual welder but is unresponsive to the resistive changes which occur across the weld during the welding operation. The system compares the current characteristics of the modeling circuit with those of the actual welding circuit and terminates the welding current in response to the true resistance change which occurs during weld formation.